What Lies Beneath

It is important to figure out how much water is on Mars – such information can help astrobiologists better understand the potential for life on Mars both in the past and present, help planners design future human settlements, and help scientists unravel the complex history of the Red Planet.

A partial view of the martian south polar ice cap (also known as the south polar layered deposits) taken by the High Resolution Stereo Camera on Mars Express, from an altitude of 269 kilometers. Click image for larger view.
Credit: ESA/DLR/FU

Because of the thin atmosphere and low air pressure, water ablates, or turns directly from ice into gas, when warmed on the surface. But Mars has a lot of ice below ground, and some believe there could be hidden reservoirs of liquid water as well. NASA and European Space Agency (ESA) spacecraft are currently using ground-penetrating radar to map what lies beneath the martian surface.

A new report describing ESA’s Mars Express radar data was just published in the journal Science, so Astrobiology Magazine’s Leslie Mullen sat down with Jeff Plaut, co-Principal investigator of the MARSIS radar instrument, to discuss the findings.


Astrobiology Magazine (AM): The MARSIS radar has detected water ice beneath the south pole of Mars. Your report in the journal Science says the ice below ground extends far beyond the ice visible on the surface, and this therefore greatly increases the known martian reservoir of water.

Jeff Plaut (JP): That is one of the important results, yes. The gamma ray and neutron measurements on Mars Odyssey had already indicated that the arctic regions of Mars are ice-rich in the upper meter or so, but we are looking much deeper.

Another one of our findings is that the polar layered deposits on the surface are very transparent to our radar waves. This suggests it’s rather clean ice, not contaminated by very much dust. Only a small fraction — maybe up to 5 or 10 percent – consists of dust. They’re predominately water ice.

AM: Is that an important finding because it improves our understanding of Mars, or does it have applications for future exploration?

This map shows the thickness of the south polar layered deposits of Mars, with purple representing the thinnest areas and red the thickest.. The total volume of ice in the layered deposits is equivalent to a water layer 11 meters deep, if spread evenly across the planet.
Credits: NASA/JPL/ASI/ESA/Univ. of Rome/MOLA Science Team/USGS

JP: There’s never been any disagreement that those deposits are mostly ice. If you’re talking about mining water ice, then maybe your job is a little easier if there’s less dust in it, but it doesn’t really change the story.

For our understanding of the processes that have produced the polar layered deposits, one gap in our knowledge was the exact composition of the layered materials. We haven’t determined the exact composition, but we have put bounds on just how much dust there might be mixed in. It turns out it’s a small amount. Other instruments weren’t able to make that measurement, so that’s an important new result.

AM: And that tells you things such as the age of the ice, or the global cycling of the ice?

JP: Among other things. These layers are believed to represent some kind of climate signature. The layers that look darker in the camera images probably have more dust in them, which means they were deposited during a climate period when the atmosphere was releasing a lot of dust in the polar regions. Whereas other layers appear very bright in the optical images, almost dust-free, so that might represent another climate period. There’s some sort of climate record in these layered sequences, and to understand what that means it’s helpful to know what the composition of those layers is, and we’re making some progress with MARSIS towards that.

AM: How much ice is there in the south polar region?

JP: The MGS MOLA data of the south pole surface topography had produced an estimate of about 1 million cubic kilometers. But with MARSIS, we’ve been able to measure down to the bottom of the ice deposit. MOLA measured the shape of the top, we measured the bottom, and that gives us a better estimate of the ice thickness. We’ve mapped the thickness of that ice in great detail over the entire deposit, and it goes from zero at the edges up to a maximum thickness of about 3.7 kilometers. We’ve discovered that the total volume is closer to 1.5 million cubic kilometers.

The upper image is a ‘radargram’ from the MARSIS instrument on ESA’s Mars Express. It shows data from the subsurface of Mars in the water-ice-rich layered deposits that surround the south pole of the planet. The lower image shows the position of the ground track of the spacecraft (indicated by a white line) on a topographic map of the area based on data from the MOLA laser altimeter on board NASA’s Mars Global Surveyor. The images are 1250 kilometers wide.
Credits: NASA/JPL/ASI/ESA/Univ. of Rome/MOLA Science Team

This volume estimate is a large fraction of the water that we know of on Mars. The largest chunk of the total water inventory of Mars, by far, is in these layered deposits. The north and the south combined have much more water than the vapor in the atmosphere, more than the amount of ice that’s been estimated in the upper meter of the surface.

A lot of our effort in studying Mars is to “follow the water.” These newly discovered ice-laden deposits, hidden beneath the surface of the southern layered deposits, is a substantial additional reservoir that has about half as much ice as the layered deposits themselves. It’s not a small addition, it’s the second largest reservoir that we know of.

AM: What do you mean by the term “ice-laden deposits?”

JP: It means they’re a mixture of ice and some other material. But that’s an interpretation of our data. The fact that we see deeply into this material suggests to us that it’s ice-rich, but right now that’s a hypothesis we need to test further. It may turn out there’s not as much ice as we think, or no ice at all. But various lines of evidence are leading us to believe there is ice.

The radar image shows the surface as a bright line. Far beneath that is another bright line, which is the bottom of the deposit where ice sits upon rocky material, or material that has much less ice. In between the top and bottom line is all ice.

In some of the radar data, the bright bottom line ends suddenly, and that tells us the nature of the boundary has changed. There’s a whole host of reasons why that boundary might become invisible to us: there may be pre-existing topography so that even though it’s a sharp boundary between icy material and the rocky material, there’s so much texture that our radar doesn’t see it because it’s easier for radar to detect smooth things. Or the minerals in the lower material may be different, or the mixture of ice and dirt in the upper materials may be different from one place to another.

AM: There are also dark circles in the radar. What are those?

Artist’s impression of water under the martian surface. If such underground aquifers do exist, humans could more easily explore and eventually colonize the Red Planet.
Credit: Medialab, ESA

JP: I call them “enigmatic deep reflectors.” We found certain areas that seem to have negative topography, or deep holes. They may be impact craters that have been filled up with material, such as dust or ice or a mixture of the two, but in a slightly different geometry or configuration than the nice organized layers above them.

These holes add to the volume measurement. If we didn’t know the hole were there, if we assumed a flat surface between data points, then there would be a smaller volume. And so those features are a major contributor to the change in the volume estimate from 1 million to about 1.5 million cubic kilometers.

AM: Could they be deposits of liquid water?

JP: No, we don’t have strong evidence for liquid water. You would need a geothermal heat source from below — a remnant of some volcanic activity — or you would need to have an insulating blanket of material on top to keep things warm enough so that the natural heat from the planet would melt the ice. But we don’t have any corroborating evidence for those scenarios. Also, the radar observation is not a distinct signature of liquid water. So there’s nothing that leads us to conclude there’s liquid water underneath the layered deposits.

Liquid water would show a strong reflection in the radar data. Water is much more highly reflective than any rock or ice material. If there were a feature such as Lake Vostok underneath the Antarctic ice sheet it would have a strong distinct signature in MARSIS, but we don’t see anything like that.

AM: What are the future plans for MARSIS?

JP: The prime phase of the Mars Express mission ended in 2005, and it is now in its extended phase. This extended phase ends this year, but ESA just decided in February to continue the mission at least until May of 2009.

This map shows the topography of the south polar region of Mars, including topography buried by thick deposits of icy material. The black line shows the boundary of the south polar layered deposits. Elevation values within the black outline show the topography at the boundary between the layered deposits and the underlying material, an interface known as the ‘bed’ of the deposits. Purple and blue represent the lowest elevations, and orange and red the highest. The total range of elevation shown is about 5 kilometers.
Credits: NASA/JPL/ASI/ESA/Univ. of Rome/MOLA Science Team/USGS

This year we’ll be looking at the middle northern latitudes. The northern plains have been hypothesized over the years as being the site of a former ocean. When we first began in 2005, we detected a feature in the northern plains that we believe is a large buried impact crater that may have icy material filling it. There also are deposits associated with volcanoes. There are outflow deposits — the large channels that drain Valles Marineris and some of the other areas near the equator spread out onto the northern plains. So we have a lot of interesting targets to look at there.

Then, towards the end of 2007, we’ll come back to the southern hemisphere and fill in gaps in our data, looking at areas we haven’t seen, targeting some of the places where we’ve seen some interesting features, putting the radar into some different operating modes, using some of the other frequencies to get as much detail in our observations as we can.

AM: Is the orbit of Mars Express the reason why MARSIS has focused almost exclusively on the south pole so far?

JP: Yes. The orbit is such that the spacecraft visits Mars three times a day, and that close approach is at a certain latitude, and that latitude very slowly drifts. When we began, that latitude was in the north, but it was a very short season for us because we operate at night. We get the best quality data when we’re on the night side. Later in 2005 the orbit drifted back into the night, and by that time it had drifted to the deep southern hemisphere. So we investigate wherever the orbit takes us.

AM: How long could Mars Express keep going?

JP: They estimate the fuel reserves would allow between 5 and 20 more years. The precise amount of remaining fuel isn’t known; it’s difficult to measure that. There are other systems on the spacecraft that can degrade or fail, but right now we’re looking at a healthy spacecraft that could last for a number of years.


Related Web Sites

Martian Poles in the Swiss Alps
Martian Water, Past and Present
Sounding Out Mars
MARSIS
SHARAD